I believe it includes everything outside the PHP code itself or MySQL. So it would include reading files from disk or network requests.
I’m not sure off the top of my head but I think the MySQL section only includes usage from Q2A functions (qa_db_*). So if a plugin calls MySQL using the native mysqli_* functions it may count in “Other” too.
By the way if you want to get detailed performance info I’d recommend trying the Xdebug profiler alongside webgrind (or kcachegrind). It will show you exactly which functions take the most time.